The present invention relates to a MOSFET and a manufacturing method thereof, and particularly to a MOSFET having an SOI structure.
In an SOI device, a semiconductor substrate layer and a transistor forming layer (i.e., SOI layer) formed thereon are insulated and separated from each other by an embedded oxide film. It is thus possible to perform insulation and separation between adjacent elements easily. Further, since no parasitic thyristor is formed through the semiconductor substrate layer, a latchup phenomenon can be prevented from occurring. Building a transistor into the SOI layer becomes effective in suppressing a so-called short channel effect that the characteristic of the transistor is deteriorated with its miniaturization. Further, since the junction capacitance of a transistor formed in an SOI structure is smaller than that of a transistor having a bulk structure, a high-speed operation is enabled. Thus, the transistor having the SOI structure has been expected as a device that has many excellent characteristics and is capable of achieving speeding-up and low power consumption as compared with a semiconductor element formed in a conventional bulk substrate.
In a MOSFET (hereinafter described as “SOI-MOSFET”) having an SOI structure, high-concentration impurity doping is done on a channel region to prevent a reduction in the threshold voltage Vt due to a short channel effect and the occurrence of a punchthrough phenomenon. However, the high-concentration doping to the channel region yields a reduction in the mobility of carriers, thus resulting in the harmful effects of the saturation current of the MOSFET being reduced. In order to solve this problem, relatively high-concentration so-called Halo layers of a conductivity type different from that of source/drain regions are formed so as to surround the source and drain regions and extension regions provided at the ends of these in a state in which the impurity concentration of the channel region is kept low, thereby avoiding the occurrence of the punchthrough phenomenon or the like while the saturation current is being ensured (refer to a patent document 1 (Japanese Unexamined Patent Publication No. 2003-282879)).
In the MOSFET having the conventional structure, the Halo layers are formed at the ends of both of the drain and source regions in terms of ease of its manufacture. When, however, the relatively high-concentration layer is formed at the drain region end, an increase in the offleak current at a drain junction is incurred. As the required performance of the MOSFET, there may be mentioned a high saturation current, high input follow-up and a low offleak current. However, all the measures for performing improvements in saturation current characteristics cause even an increase in the offleak current, thus encountering difficulties in balancing these with each other at high levels.